True, and that often occurs to me. However sometimes when the problem seems to be poltergeist type activity, it would make sense to set up cameras in empty rooms, especially if you have noticed things being rearranged during your absence or overnight and you want to find out what's causing it.
Obviously there are some cases where a home security system has sent an alert to the home owner to let them know that movement has been detected, so presumably that is why they have checked the recording.
It does make you wonder how many things get seen by security cameras but never get noticed because the footage is never checked.
Slightly more puzzling to me are the occasions where people have posted something they have filmed on their phone while they are experiencing something creepy and the phone camera is filming their own face as they are reacting rather than capturing their environment. It's as if they want to show people how scared they are instead of possibly capturing the 'ghost' on camera.